1. Field of the Invention
The present invention relates to membrane potentiometer keyswitches for use with a domestic appliance control. More specifically, the field of the invention is that of fault detecting membrane potentiometer keyswitches.
2. Prior Art
Membrane potentiometer switches typically include a resistive element located beneath a flexible membrane keypad. The keypad may be pressed at any point above the resistive element, creating a voltage signal which varies depending on the point where the keypad is pressed Keyswitches of this type are especially suited for use in electronic controls for domestic appliances, such as electric ranges. An example of a keyswitch may be found in U.S. Pat. No. 4,901,074, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference.
On existing membrane potentiometer keyswitches, the switch is open-circuited when the keypad is not pressed, so no voltage signal is provided at the switch output. If the resistive element fails for some reason, the switch will be open-circuited even when the keypad is pressed. The key will not generate a voltage signal at the switch output, and the control will not recognize that the user has attempted to enter a command. This could be particularly dangerous if the failed keyswitch was the "off" switch on an electric surface element of an electric range. In this case, a range operator could not turn off the surface element.
Membrane potentiometer switches are used as input devices on various types of controls, including electronic controls for domestic appliances. The advantage of this type of switch is that it permits the user to simultaneously input a command for an action along with the magnitude of the action desired. For example, the operator of an electric range could turn on a selected surface unit to a "medium" heat level simply by pressing the appropriate switch for the selected surface unit at the appropriate point for "medium" heat. Another benefit of using this type of switch on a cooking appliance is that the switch has a smooth surface, permitting easy cleaning of the appliance.
A prior art membrane potentiometer keyswitch is shown in FIG. 3, and includes resistive element 6 which is connected between direct current voltage source 8 and ground 10, creating a voltage drop across resistive element 6. Conductive wiper 12 is spaced away from resistive element 6 and is physically attached to a flexible membrane keypad (not shown). Wiper 12 is also electrically connected to switch output line 14. The flexible membrane keypad typically includes some type of decorative overlay which indicates the proper point to press for a desired magnitude of action.
When the flexible membrane of the keypad is pressed, wiper 12 contacts resistive element 6 at the point pressed, and provides a voltage signal to switch output line 14. The magnitude of the voltage signal depends on the resistance of the portion of resistive element 6 between the point of contact with wiper 12 and ground potential. Switch output line 14 delivers the voltage signal to analog-to-digital (A/D) converter and processor 16 which translates the observed voltage into a command signal for the appliance.
However, processor 16 cannot distinguish between the case when the keypad is not depressed, and the case when switch output line 14 is severed and the switch is inoperative. For example, if a surface heating element of a range is activated by such a keyswitch and a cooking pan was dropped so as to render the keyswitch inoperative, there would be no way of turning off the heating element by conventionally pressing the control panel keyswitches.
What is needed is a fault detecting membrane potentiometer keyswitch for a range or other appliance which prevents locking the appliance in an activated condition.
Also needed is a membrane potentiometer keyswitch which indicates when the switch is faulty.
Another need is for a membrane potentiometer keyswitch which provides distinct indications of non-activated and inoperative switches.